1. Field of the Invention
The present general inventive concept relates to an image forming apparatus, and more particularly to an image forming apparatus to shorten a warm-up time so as to effectively fuse a visible image formed on a printing medium by a developing device.
2. Description of the Related Art
Generally, an image forming apparatus is an apparatus that prints an image on a printing medium, e.g., paper, according to an input image signal. As one example of the image forming apparatus, an electrophotographic image forming apparatus is configured such that a light beam is scanned to a photosensitive member charged with an electric potential to form an electrostatic latent image on an outer peripheral surface of the photosensitive member, the electrostatic latent image is developed into a visible image by supplying yellow, magenta, cyan and black developer to the electrostatic latent image, and the visible image is transferred and fused onto paper.
The electrophotographic image forming apparatus is provided with a fusing device for fusing the visible image formed on paper by a developing device containing developer of four colors by applying heat and pressure to the visible image. The fusing device includes a heating unit having a heat source therein and a pressing unit pressing the heating unit to form a fusing nip between the pressing unit and the heating unit.
The heat source provided in the heating unit is classified as an indirect heating type or a direct heating type. The image fusing method using the indirect heating type heat source is to fuse the visible image to the paper passing through the fusing nip by using radiant heat from a halogen lamp provided inside the heating unit. The image fusing method using the direct heating type heat source is to fuse the visible image to the paper passing through the fusing nip by using direct heat transfer from a resistance heating element provided inside the heating unit.
However, the conventional image forming apparatus structured to fuse the visible image to the paper, using the fusing device provided with the indirect heating type heat source, has the problem that heat loss is generated while the radiant heat from the halogen lamp is transferred to the paper and thus a warm-up time required to reach a predetermined fusing temperature for fusing the visible image increases. Also, a resistance in an early stage of heating is low, and it causes the halogen lamp to flicker.
Also, the conventional image forming apparatus structured to fuse the visible image to the paper using the fusing device provided with the direct heating type heat source has the problem that the fusing belt should be preheated. This is because only a portion of the fusing belt contacting the resistance heating element is heated and the heat is transferred to a non-heated portion of the fusing belt which is not in contact with the resistance heating element. Therefore, the warm-up time required to reach a predetermined fusing temperature for fusing the visible image increases.